Concentrating and refining formaldehyde solutions by distillation with hydrocarbons



ZASZAM s. Y. woN CONGENTRATING AND REFINING FORMALDEHYDE SOLUTIONS BY DISTILLATION WITH HYDROCAVRBONS med July 2v. 1945 y :IIIH Ill (NUI/UU l# Za 34 and made into valuable products.

Patented Oct. 26, 1948 s -arirlzur lorifice CONCENTRATING AND REFINING FORMAL- DEHYDE SOLUTIONS BY DISTILLATION WITH HYDROCARBONS Soon Y. Wong, Pawliuska, Okla., assignor to Skelly Oil Company, Tulsa, Okla., a corporation of Delaware Application July 27, 1945, Serial No. l607,461

Claims.

This invention relates toI the concentration and refining of dilute aqueous solutions of formaldehyde. More specifically, this invention relates to the concentration and refining of dilute aqueous solutions of formaldehyde by azeotropic distillation methods with suitable azeotropeformers hereinafter described, utilizing the phenomenon of heterogeneous azeotropism in al new, original and useful application. The invention also relates to the simultaneous concentration and'reilning of dilute aqueous solutions of formaldehyde containing impuritieasuch as non-A volatile water-soluble bodies, and/'or water-,soluble high boiling bodies. The primary object of this invention is to pro vide an economical, commercially feasible and effective process whereby dilute aqueous solutions of formaldehyde may be concentrated, refined, It is also an object of this invention to provide a process of concentrating and refining dilute aqueous solutions of formaldehyde which may' be applied advantageously and effectively to dilute solutions of a rather wide range of formaldehyde concentrations. It is a further object of this invention to provide a process whereby a solution of any desired formaldehyde concentration may be recovered essentially free from impurities associated with it in its original dilute and contamf inated state. Still another object of this invention is to provide a process whereby dilute aqueous solutions of formaldehyde may be concentrated and rened in a continuous or semi-continuous process, but more preferably by a continuous process. Other objectseand advantages of this invention will become apparent to those skilled in the art upon further reading of the disclosure and claims.

The present invention is illustrated by a drawing in which:

Fig. 1 is a vertical elevation,`partially in section. of an apparatus suitable for carrying out the invention in a substantially continuous manner; and

Fig. 2 is a similar elevation of an apparatus suitable for batch operation of the process.

Data published in the literature have shown definitely that the formaldehyde present in aqueous solutions is mainly in the form of monohydrates and polymeric hydrates, and that only a small quantityl of unhydrated monomeric form- `aldehyde exists in such aqueous solutions. It has also been concluded that the various molecular forms of these hydrates are stable and nonvolatile at normal temperatures, but at elevated temperatures, i. e., above 10 0 C., they become unstable and readily decompose, liberatingI the formaldehyde in the gaseous phase to a smaller lor greater extent, depending upon the existing conditions. Re-solution and re-hydration of the liberated gaseous formaldehyde take place readily upon contact with water, reproducing an aqueous solution of formaldehyde.

It had been discovered that if dilute aqueous solutions of formaldehyde are distilled in a suitable'fractionating column of conventional design, at temperatures above 100 C. and at the corresponding superatmospheric pressures, in the presence of a suitably selected azeotropic agent capable of forming a heterogeneous azeotrope ,with water, there are produced solutions of desired formaldehyde concentrations which are greater than the formaldehyde concentrations of the original dilute aqueous solutions. These desired formaldehyde concentrations may be, for example, 37.5 or more weight per cent of formaldehyde.

In essence, my invention utilizesthe phenomenon of heterogeneous azeotropism in a new and original application.V More specifically, a dilute aqueous solution of formaldehyde is distilled in a suitable fractionating column of conventional design in the presence of an azeotropic. agent capable of forming a heterogeneous azeotrope with water, at temperatures exceeding 100 C. and at the corresponding pressures. Under such conditions the formaldehyde hydrates present in the said dilute aqueous solution are decomposed, thus allowing ythe formaldehyde to volatilize. Furthermore, the quantity of water appearing in the iina1 distillate is limited, controlled, and regulated to azeotropic proportions bythe action of the added4 azeotropic agent which functions as the The over-all result ofthis distillation at elevated temperatures and corresponding elevated pressures in the presence of such an azeotropic agent is the production of a vapor mixture in which the weight-ratio of formaldehyde tewater is much greater than the corresponding weight-ratio of formaldehyde to water of the original dilute solution. Condensation of the binary heterogeneous azeotrope, consisting of the added azeotropic agent and water, in the presence of the liberated and gaseous formaldehyde, results in a re-absorpy tion, re-solution, and re-hydration of said gaseous formaldehyde by the aqueous portion of the distillate, and separation into' one phase comprising the azeotropic agent and another phase comprising a formaldehyde solution whose formaldehyde .waste water as residues. v

The wide applicability of this invention for the 'production of formaldehyde solutions of any desired formaldehyde concentration from dilute aqueous solutions of formaldehyde of a rather wide range of formaldehyde concentrations is evident, since the weight-ratio` of 'formaldehyde to water in the overhead vapors may be limited, controlled, and regulated to any desired value primarily by the choice of the proper azeotropic agent so as to form the required heterogeneous azeotrope with water, and, secondarlly.- by the control of the operating conditions and variables such as charge rates, heat input, reflux ratios, and temperatures.

Therefore, this invention, speaking broadly, is a process for concentrating and refining dilute aqueous solutions of formaldehyde, the said proc.- ess comprising the distiliing'of diluteaqueous solutions of formaldehyde in a, suitable fractionating column of conventional design, at temperatures above 100 C. and pressures corresponding to the temperatures of distillation, with an azeotropic agent capable of forming a heteroigeneous azeotrope with water, limiting, controlling, and regulating the -amount of water in the nal distillate primarily by the use of the added azeotropic agent, said amount of water being the quantity such that the weight-ratio of formaldehyde to water is-at the desired value, said value being greater than that of the corresponding ratio of formaldehyde to water of the original `dilute aqueous solutions, condensing the condensable overhead vapors, causing a re-solution, re-absorption, and re-hydration of the formaldehyde liberated from the said dilute aqueous solutions by the aqueous portion of the distillate, separating the obtained formaldehyde solutions from the azeotroplc agent, returning the separated azeotropic agent as reflux to the fractionating column, and withdrawing, as a distillate product, formaldehyde solutions of desired formaldehyde concentrations, which latter are greater than those of the starting dilute aqueous solutions, and withdrawing the still-residues which are composed essentially of water and nonvolatile and high boiling impurities.

'I'he preferred distillation temperatures range from lC. to 150 C., depending upon various? factors, such as the particular azeotropic agent employed, formaldehyde concentration of the dilute aqueous solution of formaldehyde, nal formaldehyde concentration of the desired product, and the optimum and most economical per cent recovery of total formaldehyde. For a given dilute aqueous solution of formaldehyde being concentrated in the presence o1' a specified azeotropic agent to a formaldehyde solution of a desired formaldehyde concentration under given operating conditions, the preferred optimum temperature is the lowest temperature at which substantially all the formaldehyde, as such, in the dilute aqueous solution is liberated from its original state of hydration and solution and ultimately recovered as a distillate product. Temperatures in excess ofthe preferred range may l pressure of water. and the partial pressure of within the range from 10 vazeotropic agent.

gaseous formaldehyde present as such, all at the prevailing temperature of distillation. For economical reasons, distillation pressures should not exceed 200 lbs. per square inch gauge. Pressures lbs. per square inch gauge to lbs. ferred. f

. The condensed azeotropic agent is preferably returned as reflux to the distillation system. Depending uponthe concentration of the dilute aqueous solution of formaldehyde being concentrated, the specific azeotropic agent used, the.

temperature of the distillation, the desired formaldehyde concentration of the formaldehyde solution withdrawn as product, and other operating conditions in existence, a reflux composed of a portion of the formaldehyde solution distillate may be used in conjunction with the azeotropic agent returned as reflux. This is to say that for any given and defined system of operation, it may be advantageous, desirable, preferable or necessary to use part of the formaldehyde solution distillate as reflux together with the The primary purpose of such reiiux is to aid in maintaining the desired weightratio of formaldehyde to water in the distillate and/or to correct an unfavorably displaced ratio to the ratio desired.

The azeotrope-formers used are hydrocarbons having from about 6 molecule. Essential requirements of normally liquid hydrocarbons containing from about 6 to 15 carbon atoms per molecule` and suitable for use as azeotropic agents, are that they must be capable of forming heterogeneous azcotropes with water and be relatively insoluble in water and/or in formaldehyde solutions, essentially non-reactive with respect to water and formaldehyde under the conditions of operation, stable at the temperatures and pressures at which they may be employed, and exist in the liquid state at normal temperatures and pressures. t

It is not desired to limit this invention to any normally liquid hydrocarbon of any specified boiling range, except to those limitations and requirements specified in the above paragraph and the appended claims, as the normally liquid hydrocarbon most advantageously to be employed will depend largely upon the formaldehyde concentration ofthe dilute aqueous formaldehyde solutions to be concentrated, the desired formaldehyde concentrations of the product, and the' particular conditions under which the process of this invention is to be conducted. Any normally liquid hydrocarbon meeting the essential requirements given in the previous paragraph, therefore, will be considered to be within the scope of this invention.

A few examples of normally liquid hydrocarbons found suitable for use as azeotropic agents in the processes of the present invention are hydrocarbons having from about six to about fifteen carbon atoms per molecule, such as: heptane, octane, nonane, l-octene, l-nonene, benper square inch gauge are preto 15 carbon atoms per' zene, toluene, xylene. ethyl benzene, propyl benzene, ethyl toluene, cyclohe'xane, methyl cyclohexane, ethyl cyclohexane, .l-cyclohexene and methyl cyclohexene. Other methods of conceness of the present invention, the following specific examples are givenfthese examplesbeing various variations of the many possible methods of conducting the process of this invention. These examples are not intended to lbe limiting, but rather merely illustrative.

EXAMPLE I (BATCH PROCESS) Six hundred milliliters of a dilute aqueous` formaldehyde solution, analyzing 20.1% by weight formaldehyde, were initially charged in-v to the lower portion of a packed fractionating column 2 (see Fig. 2), through pipe 4. An equal volume of an azeotropic agent, namely, toluene.

was also introduced, together with the dilute aqueous formaldehyde solution. through said pipe. The amount of toluene thus charged was in excess of requirements. which was done to insure the presence of a suillcient amount of liquid in the kettleor lower portion of the column 2 as the operation approached its conclusion, so as to prevent damage by overheating due to an insuillcient amount of liquid. Essentially all the toluene was returned as reflux through pipe I4 (Fig. 2) for reuse in the process, after condensation and phase separation from the formaldehyde solution obtained during the condensation. Heat was applied to the kettle 3 (lower part) of the column and the column was purged of air through a relief valve 31 installed on the end of the condenser 8. After essentially all the alrwas displaced from the column, the relief valve 31 was shut and the column 2 was allowed to reach the desired operating vtemperature and pressure. Vapors, consisting of formaldehyde and of the toluene and water in azeotropic proportions, were removed from the top of the column through pipe 6, and conducted to a water-cooled condenser 8, wherein condensation of the azeotropic vapor to again form liquid toluene and water was effected, with subsequent re-solution. re-absorption, and re-hydration of the uncondensed formaldehyde vapor in the resulting aqueous portion of the resulting condensate.

' The total condensate was then conducted through pipe I2 to a phase separator I0, wherein the toluene was separated from the obtained strongi. formaldehyde solution. The top liquid layer consisted of toluene while the bottom layer consisted of the concentrated aqueous formaldehyde solution. All ofthe toluene thus separated was returned as reflux to the column through pipe I4, along with a portion of the obtained and separated formaldehyde solution. After thencolumn 2 reached the desired temperature and presure, thirteen separate portions of the separated strong formaldehyde solution were removed from the distillation system for analysis, by withdrawal from the phase separator I0 through pipe I6. A reiiux ratio of 15:1 was used. At thetermination of the run, the kettle residue was recovered and analyzed for its formaldehyde content.

The data and results obtained in this run are summarized in Table I.

Tam.: I

Summary of data and results, Example I Azeotropic agent used...y Q-, Toluene Average kttle temp., "C 144.0 Average overhead temp., "C 131.0 Average pressure. p. s. i. g 60.4 Charge stock:

Volume ml 600. Wt. percent formaldehyde 20.1

Wv'fht Per v 1 h Fraction i\0. Mllrs Cllt.e

25 l52.5 n 25 42.9 25 43.4 2s 43.9 2t 48.5 25 44.9 25 41.4 2s 34.0 21 23.1 2t ,11.3 25 las 25 9.1 2s as 23o 1.a

To further illustrate the results obtained by the process of this invention, the following specific example is given, said example being another variation of the many possible lmethods oi conducting the process of this invention.

EXAMPLE II (BATCH PROCESS) Five hundred milliliters of( a dilute aqueous formaldehyde solution cont ining 20.6% by weight formaldehyde were initially charged into the kettle 3 of a packed fractionating column 2 (see Fig. 2) through pipe 4, Six hundred milliliters of an azeotropic agent, namely a commercial octane hydrocarbon fraction,`was also introduced, together with the dilute aqueous formaldehyde solution, through said pipe. The amount of octane hydrocarbon fraction thus charged was in excess of requirements, which was done to insure 'the prsence of a suiiicient amount of liquid in formaldehyde solution obtained during the condensat-ion. Heat was applied to the kettle 3 of the column, and the columnlwas purged of air vthrough a relief fvalve 31 installedV on the end of condenser 8." After essentially all the air was displaced irom the column, the relief valve 31 was shut and the column 2 was allowed to reach the desired operating temperature and pressure. Vapors consisting of formaldehyde and of the octane hydrocarbon and water in azeotropic proportions, were removed from the top ofthe col- .umn through pipe 6 and conducted to a watercooled condenser 8, wherein condensation of the azeotropic vaporto again form liquid octane hydrocarbon and water was effected, with subsequent re-solution, re-absorption and re-hydration of the uncondensed formaldehyde gas in the resulting aqueous portion of the resulting condensate.

The total condensate was then conducted through pipe I2 to a, phase separator I0, wherein the octane hydrocarbon was separated from the oatained strong formaldehyde solution. The upper liquid layer consisted of octane hydrocarbon while the lower liquid layer consisted of the concentrated aqueous formaldehyde solution, All of the octane' hydrocarbon thus separated was returned as iiux to the column 2 through pipe I4, along with a portion ofthe obtained and sepa.-

rated formaldehyde solution. After the column Tenu: II Summary of data and results, Example II Azeotropic agent used Commercial octane fraction aaasesssees eer-sassarese blgableeul 're further illustrate Athe results obtained by' the process of this invention, the following speclflc example ,is given, said example being one variation of the many possible methods of conducting the process of this invention in a continuous manner.

EXAMPLE III (CONTINUOUS) 6,600 milliliters of 'a dilute aqueous solution of formaldehyde containing 20.1% by weight formaldehyde were continuously charged at the rate of 825 milliliters per hour into a packed column' to which had been initially charged approximately 1,465 milliliters of the azeotropic agent toluene.` It was found by previous experimentation that the 1,465 milliliters of toluene were sufilcient to be eiective throughout the column and to maintain a toluene phase in the reboiler. Preheated formaldehyde solution ranging in temperature from about l40143 C. at a pressure of about 70 lbs. per square inch gauge was charged to the column at a point approximately threeifths of the vertical length of the column from the bottom. The distillation system was operated at an average pressure of 63 lbs. per square inch gauge with an average overhead temperature of 132 C. and a reboiler 'temperature oi' 158 C.

Suillcient` heat was supplied to the reboiler to create vaporization of the reilux. Vapors, consisting oftoluene and water in azeotropic proportions, and of formaldehyde, were removed from the top of the column and conducted to a watercooied condenser, wherein condensation o1' the i azeotropic vapor consisting of toluene and water was eilected, with resulting ,re-solution, re-absorption ,and re-hydration of the uncondensed formaldehyde vapors in theA aqueous portion of the distillate. The total condensate was then conducted to `a phase separator wherein thecondensed toluene was separated from the aqueous portion of the condensate by gravity. Eight fractions of 330 milliliters each were withdrawn, at a constant rate, for formaldehyde analysis: and in addition to these distillate fractions, eight fractions of the aqueous portion of the reboilers con- Y tents were continuously cooled, separated in another phase separator, and withdrawn. at a constant rate, vfor a formaldehyde analysis. uene separated out during this operation was suitably returned to the reboiler of the column. The data and results of thiarun are given in Table III.

Tenu III Summary of data and results, Ecample 'HI Azeotropic agent used Toluene Average reboiler temp. C 158 Average overhead temp.` C LVISR Average pressure, p. s. i. g s

Distillate Residues F'ctfm N Wt. Per wt. Per w,

V 85% omo Vi" ggf, omo

44.1 157.8 49s 1.o s o 4.3.9 156,5 o 1.a aa 44.0 151.0 49o 1.2 s 44.2 155.1 41o ce 4.a 44.1 157.8 488 1.0 4.9 44. 0 157. 0 490 1. 1 5. 4 44. 2 168. 7 .475 0. 9 4. 3 43.9 156.7 495 1.2 0.0

Total formaldehyde charged ..--grams 1408.0 Total formaldehyde recovereddo. 1302.1 Recovery of formaldehyde by weight percent-- 925.

EXAMPLE IV For a clearer understanding of the continuous operation, a complete cycle of operations will be described in connection with Fig. l. Thus', a

dilute aqueous 'solution of formaldehyde of essen-- tion and capable of formingV a heterogeneousv azeotrope vapor with the water of the dilute aqueous formaldehyde solution. Heat is applied to the mixture and as thetemperature in the fractionating column exceeds C., vapors, apparently comprising formaldehyde and an azeotrope comprising the azeotropic agent and water, are withdrawn from the top of the fractionating column 2 through pipe 6 andare condensed in condenser 8. At the temperature to which the condensate is cooled, the condensate forms an azeo.- tropic agent phase and a water-formaldehyde phase; the cooled condensate is then passed through pipe i2 into a separator I0, substantially at the vertical center thereof. The water-formaldehyde phase of desired formaldehyde concentration settles to form a lower layer and the azeotroplc agent phase rises to form an upper layer. The formaldehyde solution, i. e. the lowerlayer,

entame phase is withdrawn from reboiler 2li through pipe 2li to cooler 26 where it is cooled and then with- 10 drawn through pipe 28 to a second phase separator 30. Any azeotropic A,agent separated out therein is removed from the phase separator 30 and returned to reboller 20 through pipe 82. The

rnal residue, consisting essentially of water` and associated impurities, is withdrawn from the separator 30 through pipe 36 and Ndiscarded. Make-up azeotropic agent is added, when necessary, through pipes 36 and d.

While I have described the principles of my process for the concentration of dilute aqueous solutions of formaldehyde, it is to be understood that the illustrations and examples are merely to clarify the general mode of operation, and that this invention is not limited to specic details of temperature, pressures, weight ratios, azeotropic agents or specific apparatusexcept as defined in the appended claims. K Having described my, invention, I claimt:

1. In a process of refining and concentrating dilute aqueous solutions of formaldehyde, the

` steps of distilling, at a temperature in excess of `100 C., said dilute aqueous solutions together with a normally liquid hydrocarbon having from about 6to about 15 carbon atoms per.molecule, 35

and collecting. the overhead formaldehyde vapors and condensing them as formaldehyde solution of greater concentration than that of said dilute aqueous solutions.

2. In a process of refining and concentrating dilute aqueous solutions of formaldehyde, the steps of distilling, at a temperature in excess of 100 C., said Idilute aqueous solutions together with a normally liquid hydrocarbon having from about 6 to about 15 'carbon atoms per molecule, with- 45 drawing athe overhead vapors, condensing and cooling the said overhead vapors, thus effecting a re-solution of the formaldehyde in the aqueous portion of the condensate to form a normally liquid hydrocarbon phase and a formaldehyde-water phase of'greater formaldehyde concentration than thatofsaiddilute aqueous solutions, withdrawing the normally liquid hydrocarbon phase, and returning it to the distillation for further use. y f

3. In a process for concentrating and refining dilute aqueous solutions of formaldehyde by distilling, at a temperature in excess of 100 C., said dilute aqueous solutions in a distilling zone together with a normally liquid hydrocarbon having from about 6 to about 15 carbon atoms per molecule to form overhead vapors comprising formaldehyde, the said normally liquid hydrocarbon, and water (the latter two being in azeotropic proportions), leaving -excess water and associated impurities as a kettle product, withdrawing, condensing and cooling the saidv overhead vapors thereby causing a re-solution of the formaldehyde vapors in the aqueous portionl of .the resulting condensate, separating .the condensate mixture of normally liquid hydrocarbon and aqueous formaldehyd-e into i a formaldehyde solution layer of greater formaldehyde concentration 'than that of said dilute aqueous solutions, and a normally liquid hydrocarbon layer, withdrawing the norqonducted from frac, y5 tionator 2 through pipe i8 to reboiler 20. vapors generated in reboiler 20 are passed to fractionator i: through pipe 22. Some ofthe aqueous residue mally liquid hydrocarbon layer. and returning it g directly to the said distillation zone.

4. A continuous process for' concentrating and renning dilute laqueous solutions of formaldehyde by distilling at a temperature in excess of 100 .C., said dilute aqueous solutions in a distilling zone together` with anormally liquid hydrocarbon having from about 6 to about 15 carbon atoms per molecule to form overhead vapors comprising the normally liquidhydrocarbonand water in azeotroplc proportions and formaldehyde, continuously withdrawing as a kettle product excess.

'ing the normallyl liquid hydrocarbon., layer and returning i-t directly to the said distillation zone as reflux and ywithdrawing the formaldehyde solution layer of greater formaldehyde concentration than that of said dilute aqueous solutions from the distillation process.

5. In a process for concentrating and refining a'dilute aqueous solution of formaldehyde, the steps of distilling, at a temperature above 100 C., said solution together with a normally 4liquid lw- -drocarbon having from about 6 to about l5 carbon atoms per molecule and capable of forming a heterogeneous azeotrope with water, leaving as a kettle product excess water and associated impurities, regulating the amount of water in the overhead vapors by the use of said normally liquid hydrocarbon, condensing and cooling 'the normally liquid hydrocarbon-water-formaldehyde overhead vapors which effect a re-solution of the unconde'nsed formaldehyde in the aqueous portion of the condensate, separating the normally liquid hydrocarbon-water condensate intoa concentrated formaldehyde solution layer and a normally liquid hydrocarbon layer, withdrawing the normally liquid hydrocarbon layer and returning it to the distillation as reflux.

6. In a process vfor concentrating and refining dilute aqueous solutions of formaldehyde, the

' azeotrope with water, leaving a kettle product of excess water and associated impurities, regulating the amount of, water in the overhead vapors by the 4use of the said normsuy nqiua hydrocarbon, condensingv `and cooling the normally liquid hydrocarbon-water-formaldehyde overhead vapors. .thus effecting a re-solution of the uncondensed formaldehyde in the aqueous portion of the condensate, separating the normally liquid hydrocarbon-formaldehyde-water condensate into a formaldehyde solixtion layer of greater formaldehyde concentration than the original dilute aqueous solutions of formaldehyde and a normally liquid hydrocarbon layer, withdrawing the nor--4 mally liquid hydrocarbon layer, and returning i-t directly to the said distillation zone as reflux,` and withdrawing and recovering the said formaldehyde solution layer from the distillation process.

7. In a process for' concentrating and renning dilute aqueous solutions of formaldehyde, the steps oi. distilling, at a temperature greater than 100 C., said solutions in a distillation zone t0- gether with a normally liquid hydrocarbon having from about 6 to about 15 carbon atoms per molecule and which is substantially immisclble with water, leaving a kettle product of excess water 'and associated impurities, regulating the amount of water in the overhead vapors by the use of saidsnormally liquid hydrocarbon, condensing and cooling the normally liquid hydrocarbon-water-formaldehyde loverhead vapors,

thus effecting a re-solution of the uncondensed formaldehyde in the aqueous portion of the condensate, separating the normally liquid hydrocarbon formaldehyde-water condensate into La formaldehyde solution layer of greater formaldehyde concentration than'the original'lilute aqueous solutions of formaldehyde and a normally liquid hydrocarbon layer, withdrawing the normally liquid hydrocarbon layer and returning it to the said distillation zone as reflux, withdrawing and recovering the said formaldehyde solution layer from the distillation process.

8. In a continuous process "for the concentra- Vtion of a dilute aqueous solution of formaldehyde the steps of distilling said solution in a distillation zone together with a normally liquid hydrocarbon having from about 6 to about 15 carbon atoms per molecule and capable of forming a heterogeneous azeotrope with water, at a temperature above 100 C. and at superatmospheric pressure, to form overhead vapors, continuously withdrawing the overhead vapors, continuously` of greater formaldehyde concentration than the d said dilute aqueous solution of formaldehyde.

9. In a continuous process for the concentration of a dilute aqueous solution of formaldehyde, the steps of distilling said solution in a distillation zone together with a normally liquid hydrocarbon having from about to about 15 carbon atoms per molecule and capable of forming a heterogeneous azeotrope With-water, at a temperature above 100 C. and at superatmospheric pressure, to form overhead vapors, continuously withdrawing the overhead vapors, continuously withdrawing the kettle product ofv excess water and associated impurities, regulating the amount of water in the overhead vapors by the use of the said normally liquid hydrocarbon, condensing and cooling the overhead vapors, thus effecting a re-solution of the uncondensed formaldehyde in the aqueous portion of the condensate, separating the normally liquid hydrocarbon-water formaldehyde condensate into a concentrated` formaldehyde solution layer and a normally liquid hydrocarbon layer, continuously withdrawing a portion of the concentrated formaldehydesolution layer and returning it with the normally liquid hydrocarbon layer to the distillation zone as reflux.

10. In a continuous process for the concentration of a dilute aqueous solution of formal- 12 dehyde. the steps of distilling said solution in a distillation zone together with a normally liquid hydrocarbon having from Vabout 6 to about 15 carbon atoms per molecule and capable of forming a heterogeneous azeotrope with water, at a temperature above 100 C. and at superatmospheric pressure, to form overhead vapors in which the water is limited and controlled bythe said ,portion of the condensate, separating the normally liquid hydrocarbon-water-formaldehyde condensate intofa formaldehyde solution layer and,

normally liquid hydrocarbon layer, continuously withdrawing the formaldehyde solution produced containing an amount of formaldehyde of not less than 37.5% by weight of formaldehyde.

-11. T he process as defined in claim 10 in which the dilute aqueous solution of/formaldehyde contains less than 37.5% formaldehyde by weight.

12. In a continuous process for the concentration of a dilute aqueous solution of formaldehyde, the steps of distilling said solution in a distillation zone together with a normally liquid hydrocarbon having from about 6 to about 15 carbon atoms per molecule and capable of forming a heterogeneous azeotrope with water in which both the dilute aqueous solution of formaldehyde and a normally liquid hydrocarbon volatilize from the mixture at a temperature of from 102 C. to 150 C. and at superatmospheric pressure, to form overhead vapors, continuously withdrawing the overhead vapors, in which the water is limited and controlled by the normally liquid hydrocarbon, continuously withdrawing the kettle product of excess water and associated impurities, condensing and cooling the overhead vapors, thus effecting a re-solution of the uncondensed formaldehyde in the aqueous portion of the condensate, separating the normally liquid hydrocarbon-water-formaldehyde condensate into a concentrated formaldehyde solution lower layer and a, normally liquid hydrocarbon upper layer, continuously withdrawing the .normally liquid hydrocarbon upper layer and returning it to said vdistillation zone as reflux and continuously withdrawing the formaldehyde solution lower layer of desired concentration. y

13. The process as defined in claim 8 in which the normally liquid hydrocarbon is toluene.

14. The process as defined in claim 8 in which the normally liquid hydrocarbon is methylcyclohexane.

15. The process as defined in claim 8 in which the normally liquid hydrocarbon is octane.

SOON Y. WONG.

REFERENCES CITED Thefollowing references are of record in the file of this patent:

479,255 Great Britain Feb. 2, 1938 

